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La Furia Roja got the better of Australia 3-0 at the Arena da Baixada in Curitiba in their final Group B clash on Monday.
Mata scored the reigning European and world champions’ third goal of the match with eight minutes of normal time remaining.
David Villa opened the scoring for Spain in the 36th minute, and Chelsea striker Fernando Torres doubled the advantage on 69 minutes.
However, after the match, Mata refused to get too excited and told reporters: ‘Whatever the result, we weren’t going to be happy’.
The 26-year-old former Chelsea star also hailed Villa after he scored on his final appearance for Spain.
‘Villa, for me, is unassailable for Spain,’ said Mata.
The Belgium international joined the Red Devils for £27.5 million, but failed to make an impact at Old Trafford.
‘The last thing we wanted was to make him my first signing as United manager and the only signing of the summer’, Moyes wrote in The Times. "That was always going to bring too great a spotlight on him.
‘But by the time we got to deadline day the club hadn’t been able to close a couple of big deals and there was no choice.’
Meanwhile, new United manager Louis van Gaal will be able to call upon England internationals Wayne Rooney, Phil Jones, Chris Smalling and Danny Welbeck for the team’s summer tour of the USA.
With England due to fly back from Brazil after their final game against Costa Rica on Tuesday, the quartet will be able to join the rest of the Red Devils squad for their departure to Stateside on July 17, according to The Telegraph.
Explaining the importance of pre-season tours of the US, United's assistant manager Ryan Giggs told the club’s official website: ‘Obviously the facilities are good, all of the lads love going to the States because you can chill out in the day, do a bit of shopping and relax when you have a bit of downtime. You don’t get pestered like you might in other places’.